The new system has provoked privacy concerns, because it allows members to search for information on Facebook such as "single women in London". Graph Search returns a list of members who meet the criteria and set their privacy setting such that whoever entered the search is allowed to see their profile.

Facebook today said it had taken special measures to ensure that teenagers were not exposed to potentially dangerous strangers.

"As with all of our products, we designed Graph Search to take into account the unique needs of teens on Facebook," it said in a blog post.

In particular, Graph Search follows the existing rule on Facebook that teenagers are not allowed to make their profile visible to anyone more distant than a friend of a friend.

"On Facebook, many things teens are likely to do - such as adding information to their timelines or sharing status updates - can only be shared with a maximum of Friends of Friends," Facebook said.

Extra care has been take over information that could be useful to predatory adults, too, it added.

"For certain searches that could help to identify a young person by age or by their location, results will only show to that person's Friends, or Friends of Friends who are also between the age of 13-17."

Facebook allows anyone to have an account if they ar 13 or older. According to Consumer Reports, a American consumer watchdog, millions of underage childen have entered false information to join the social network.

As well as highlighting its child protection features, Facebook called on members to review their profile to ensure they are comfortable with what others could see via Graph Search. It said they could remove their name from photos they had been "tagged" in or change setting to limit who could view the "About Me" section of their profile.

At the launch of Graph Search in January, Mark Zuckerberg was at pains to emphasise that privacy had been a central consideration in its design. Members given early access to the system quickly found potentially troubling uses for the system, however.

For instance, London-based developer Tom Scott created a widely shared blog to highlight the potential embarrassment or danger of Graph Search queries including "married people who like prostitutes", "family members of people who live in China and like Falun Gong [the banned sect]" and "Islamic men interested in men who live in Tehran, Iran".

Graph Search is currently only available in English. Facebook is slowly making it available to members who sign up to a waiting list.